rust•armanriazi•let•const
If variables defined with let are immutable, then why does Rust include a
const keyword? The short answer is that data behind let can change. Rust allows types to have an apparently contradictory property of interior mutability.
Some types such as std:sync::Arc and std:rc::Rc present an immutable façade, yet change their internal state over time. In the case of those two types, these increment a reference count as references to those are made and decrement that count when those references expire.
At the level of the compiler, let relates more to aliasing than immutability. Aliasing in compiler terminology refers to having multiple references to the same location in memory at the same time. Read-only references (borrows) to variables declared with let can alias the same data. Read-write references (mutable borrows) are guaranteed to never alias data.